A wide range of methods and apparatus are known in the art for processing biologically degradable waste products to provide a clarified and safe effluent for return to nature.
Basically, biological sewage treatment is a process in which microorganisms metabolize waste products producing gaseous and soluble molecular end products, and additional bacterial cells. The rapidity and effectiveness of the process is aided by increasing the quantity of bacterial cells, or standing crop, available to carry out the metabolism. The concentrating and holding of the organisms in the system has in the prior art been obtained by one of several physical or biological techniques including flocculation, sedimentation and provision of large surface areas for bacterial multiplication.
Two widely known and conventional techniques for treating biodegradable waste products are the activated sludge and trickling filter processes. In an activated sludge system, contact of organisms with waste is brought about by mixing sewage with sludge, i.e., a concentrated solid containing floes of organisms, to form the mixed liquor and then subjecting the mixture to strong agitation and the introduction of large volumes of air to improve oxygen transfer. In trickling filter systems, the biological organisms are not present in or submerged in an aqueous medium but are present in the form of exposed growth on the surface of suitable base objects in contact with the atmosphere and reaction is brought about by flowing the waste in relatively thin streams in direct contact with the biological slimes.
A submerged, attached biological wastewater treatment process is described in U.S. Pat. Nos. 3,966,599, 3,966,608 and 3,972,965, which patents are assigned to the assignee of the present invention. An integral feature of the submerged, attached growth, biological wastewater treatment process is the continual growth of organisms that periodically detach or "sluff off" the media surfaces. These clumps of biological solids are directed out of the treatment tank and are separated from the treated liquid by a subsequent clarification process. The media is designed to provide the maximum surface area for attached biological growth in a particular treatment tank volume, commonly referred to as the surface area density of a media, and to induce turbulence and mixing in liquid substrates carrying food, trace nutrients and oxygen to the organisms.
The aggregation of biomass within the media and its sluffing off of sheet surfaces are valuable characteristics because these relatively dense solids easily settle during the clarification step. However, these biomass aggregations tend to clog the media. A high surface density of the media is also a valuable characteristic because it will support a greater population of organisms that feed on waste. In general, a biological treatment process is rendered more stable and efficient when the food-to-micro-organism ratio is relatively low. Unfortunately, the combination of these two characteristics, maximum biological growth and high surface area density medias, often causes a problem with clogging.
Medias currently being used in this application include spaced apart, alternately reversed (criss-crossed) corrugated sheets. These medias create enclosed spaces subject to clogging and biomass occlusions when submerged in wastewater, particularly at the point at which the criss-crossed corrugations come into contact. These medias are aggregated in treatment tanks by stacking discrete blocks into a large and continuous pattern. The planes at which these blocks interface frequently cause additional clogging. Particularly in highly loaded treatment plants, where the thickness and profusion of attached biomass is relatively great, the clogging potential requires more open space within the media. This greater void volume reduces the overall surface area density of the media and the population of organisms that it can support.
Medias consisting of relatively large diameter conduits or tubular flow-through channels have also been adapted for the submerged, attached growth, biological wastewater treatment application. While these medias tend to solve the clogging problem, they typically fail to induce the turbulence and mixing required for the efficient operation of the mass transfer process. Such high void designs do not produce a high surface area density. Also, in a tube, the buildup of biomass inherently reduces the active surface area.
These enclosed medias may only be applied within a wastewater treatment process that uses a vertical flow circulation. As a result, the submerged attached growth process has not been combined with other common wastewater treatment processes, such as oxidation ditches having horizontal or "plug" flow regimes.
There is a need for improved medias that maximize surface area density within the treatment vessel while minimizing clogging from the biomass mass solids produced in the treatment process.